Journal of Asian Earth Sciences 83 (2014) 1–12

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Journal of Asian Earth Sciences

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Microfacies and depositional environments of the Late OrdovicianLianglitage Formation at the Tazhong Uplift in the Tarim Basinof Northwest China

1367-9120/$ - see front matter � 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jseaes.2014.01.002

⇑ Corresponding author. Tel.: +86 01062306209.E-mail address: lincs@cugb.edu.cn (C. Lin).

Da Gao a, Changsong Lin b,⇑, Haijun Yang c, Fanfan Zuo a, Zhenzhong Cai c, Lijuan Zhang c, Jingyan Liu a,Hong Li a

a School of Energy Resources, China University of Geosciences, Beijing 100083, Chinab School of Ocean Sciences and Resources, China University of Geosciences, Beijing 100083, Chinac Research Institute of Petroleum Exploration and Production, Tarim Oilfield Company, PetroChina, Korla 841000, China

a r t i c l e i n f o

Article history:Received 28 March 2013Received in revised form 26 December 2013Accepted 5 January 2014Available online 13 January 2014

Keywords:MicrofaciesDepositional environmentHigh-frequency sequenceCarbonate platformLianglitage FormationTarim Basin

a b s t r a c t

The Late Ordovician Lianglitage Formation comprises 13 microfacies (Mf1–Mf13) that were deposited ona carbonate platform at the Tazhong Uplift of the Tarim Basin in Northwest China. Each type of microf-acies indicates a specific depositional environment with a certain level of wave energy. Four primarygroups of microfacies associations (MA1–MA4) were determined. These associations represent differentdepositional facies, including reef-shoal facies in the platform margin (MA1), carbonate sand shoal facies(MA2) and oncoid shoal (MA3) on open platforms, and lagoon and tidal flat facies (MA4) in the platforminterior. Each microfacies association was generated in a fourth-order sedimentary sequence developingwithin third-order sequences (SQ1, SQ2, and SQ3, from bottom to top), showing a shallowing-upwardtrend. High-frequency sequences and facies correlation between wells suggests that the reef-shoal faciesmore successively developed in the southeastern part of the platform margin, and high-energy microfa-cies were more strictly confined by the top boundary of fourth-order sequences in the northwestern partof the platform. The highstand systems tract (HST) of the SQ2 is characterized by reef-shoals that devel-oped along the platform margin and tidal flats and lagoons that developed in the platform interior, whilethe SQ3 is characterized by the oncoid shoal facies that generally developed on the uplift due to a region-ally extensive transgression that occurred during the latter part of the Late Ordovician. The results of thisstudy can be used for investigating the development and distribution of potential reservoirs; the reser-voirs in southeastern part of the platform margin may be of premium quality because the high-energymicrofacies were best preserved there.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction Ordovician (Chen et al., 1999; Gu et al., 2005; Yang et al., 2007,

Carbonate microfacies and depositional environments analysis,as the fundamental work in sedimentology and reservoir geology,have long been of great scientific interest (Bauer et al., 2002;Flügel, 2004; Ghabeishavi et al., 2010; Adabi et al., 2010). Under-standing the patterns of microfacies successions and their con-straint by high-frequency sequences is critical for predictingfavorable reservoirs (Saller et al., 1999; Ronchi et al., 2010;Al-Awwad and Collins, 2013). The Late Ordovician LianglitageFormation has been considered as an important carbonate reser-voir at the Tazhong Uplift of the Tarim Basin (Zhou et al., 2006;Wang et al., 2007) (Fig. 1). Large hydrocarbon reserves now residein reefs and carbonate sand shoals in the margin of the rimmedplatform that developed during the early part of the Late

2011, 2010b) (Fig. 3). Studies on the facies composition and distri-bution have therefore intensified over the past decade (Liu et al.,2006, 2012; Chen et al., 2009; Wang et al., 2010). However, thevariation and associations of the microfacies on the LianglitageFormation have not been systematically studied, and the controlof high-frequency sequence on reservoirs has not been thoroughlyrevealed. Thus further exploration of potential hydrocarbonreservoirs in the Lianglitage Formation of the Tazhong Uplift hasbeen restricted. This study was aimed to analyze local microfacies,define the high-frequency stratigraphic framework, reconstructthe sedimentary environment of the Lianglitage Formation, andexplore the distribution of favorable reservoirs.

2. Geological setting

The northwest-to-southeast–striking Tazhong Uplift, with anarea of approximately 2.75 � 104 km2, is located in the middle of

Fig. 1. Map of the study area and the tectonic framework of the Tazhong Uplift (Modified from Yang et al., 2010a,b). The Tazhong Uplift is located in the center of the TarimBasin (B).

Fig. 2. Composite stratigraphic section of the Ordovician at the Tazhong Uplift in the Tarim Basin (Modified from Lin et al., 2012b).

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the central uplift belt of the Tarim Basin (Fig. 1B). It lies betweenthe Manjiaer Depression and the Tangguzibasi Depression, where-by the Tazhong No. 1 fault and the south fault belt serve as

boundaries (Fig. 1C). The Tazhong Uplift sustained a series of tec-tonic movements during the Ordovician (He, 1995; Jia, 1997; Linet al., 2009, 2011); as a result several unconformities (RSU1,

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RSU2, RSU3, RSU5, and RSU6, following Lin et al., 2012b) wereformed as boundaries of the formations in the Ordovician system(Figs. 2 and 3).

The Tazhong area maintained a stable carbonate platform fromthe Cambrian to the Early Ordovician (Gao and Fan, 2012; Linet al., 2011, 2012a,b). The closure of the North Kunlun Oceansince the Middle Ordovician caused the rise of the Tazhong Uplift(Lin et al., 2011, 2012c), and a low-angle unconformity (RSU3)developed as the bottom boundary of the Lianglitage Formation.Subsequent development of the carbonate platform system ofthe Lianglitage Formation in the Late Ordovician was constrainedby paleouplift landforms, such that the platform margin was re-stricted by the marginal fault (Tazhong No. 1 fault) of the paleo-uplift (Fig. 3) (Liu and Wang, 2005; Peng et al., 2009; Lin et al.,2009; Ren et al., 2012). Large-scale transgression in the latter partof the Late Ordovician led the carbonate platform to drown (Yanget al., 2010b; Yu et al., 2011), with the sedimentary environmenttransiting to a deepwater continental shelf and basin (Lin et al.,2012b) where the thick, mudstone-dominated strata of the Sangt-amu Formation formed. Therefore, the top boundary of the Liangl-itage Formation is a drowning unconformity (RSU5). The end ofthe Ordovician witnessed the intense deformation of the TazhongUplift and the significant truncation of the Upper Ordovician un-der the extensive angular unconformity (RSU6) due to the finalcollision and extrusion of the North Kunlun block and a globalsea-level drop (Yu et al., 2001; Bao et al., 2006; Gao et al.,2007; Lin et al., 2011, 2012c; Zhou et al., 2011). Since then, mostof structural patterns of the Tazhong Uplift have been preserved(Fig. 1C).

The Lianglitage Formation is confined between a drowningunconformity (RSU5) at the top and a low-angle unconformity(RSU3) at the bottom (Yang et al., 2010b), giving the formation athickness of 120–880 m in the Tazhong Uplift (Yang et al., 2007)(Figs. 3 and 4). The characteristics of main lithofacies and associa-tions are presented in Fig. 3. According to these features, the for-mation can be divided into three sections which are (1) theargillaceous limestone section (150–550 m), (2) the grainstone sec-tion (100–300 m), and (3) the argillaceous-striped limestone sec-tion (0–150 m) (Yang et al., 2000, 2007) (Fig. 4).

Fig. 3. Three-dimensional (3D) seismic profile showing the carbonate platform developLLTG Fm: Lianglitage Formation, Upper Ordovician; YS Fm: Yingshan Formation, LowXiaqiulitage Formation; Upper Cambrian.

3. Materials and methods

More than 650 m of cores and 1000 m of well-cuttings from 12wells in the Tazhong Uplift were studied, with about 700 thin sec-tions being prepared. The depositional textures, grain types, fossils,and early diagenetic features were described by using a polarizingmicroscope, and the abundance of argillaceous content and differ-ent types of grains and fossils was visually estimated.

The classification of carbonate rocks followed the nomenclatureof Dunham (1962) and Embry and Klovan (1971). Sedimentarymodels built by Wilson (1975) and Flügel (2004) were referredto for the sedimentary models of the Lianglitage Formation. For se-quence stratigraphic interpretation, the concepts and modelsdeveloped by Posamentier et al. (1988), Catuneanu (2006) andTucker and Wright (1990) were consulted.

4. Depositional sequences

The Lianglitage Formation can be divided into three deposi-tional sequences from seismic and well-logging data (Figs. 3 and4). The two lower sequences (SQ1 and SQ2), which contain bothtransgressive systems tract (TST) and highstand systems tract(HST), can be identified by two separated sets of gentle prograda-tional clinoforms on the platform margin (Fig. 3). The upper se-quence (SQ3) is considered to be a drowning-upward sequencein which only TST appears (Gao et al., 2007; Yang et al., 2010a).

The lowest part of the Lianglitage Formation is featured bybrown gray thin-bedded argillaceous limestone which makes upthe TST of SQ1. Gray thick-bedded wackestones and packstonessucceed and are considered to be the HST of SQ1. The upper partof the argillaceous limestone section shows an increasing trendin argillaceous content, and it is regarded as the TST of SQ2. Themiddle section (the grainstone section), roughly corresponding tothe HST of SQ2, primarily comprises medium- and thick-beddedpackstones, grainstones, and boundstones, and can be easily recog-nized from the gamma ray curve. Marly oncoid limestones withargillaceous stripes are the prevailing lithofacies of the upper mostpart of the formation. The increase in argillaceous content toward

ing during the Late Ordovician. STM Fm: Sangtamu Formation, Upper Ordovician;er-Middle Ordovician; PLB Fm: Penglaiba Formation, Lower Ordovician; XQ Fm:

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the top reflects the drowning process of the carbonate platform, sothis section consists of the drowning-upward sequence (SQ3).

Most of the wells in our study area were completed in the grain-stone section due to its best reservoir properties and thus we fo-cused on investigating the sedimentary information bounded bySQ3 and the HST of SQ2.

5. Microfacies analysis

5.1. Microfacies characteristics and environment interpretation

From the logging of cores and thin sections, a scheme of 13microfacies (Mf1–Mf13) were defined based on depositional attri-butes and early diagenetic features. The typical features of thesemicrofacies are shown in Figs. 5–7, and the detailed descriptionand interpretation of each type of microfacies is presented inTable 1.

Mf1 to Mf3 are grainstone containing abundant sand-sized,well-sorted, and well-rounded intraclasts, ooids, and various typesof fossils including mollusks, crinoids, algae, and bryozoans(Figs. 5A and 6A–D). The relatively high level of fabric maturityin these microfacies suggests that they were formed on theplatform margin where the wave energy is high. The abundance

Fig. 4. Sequences and lithofacies of the Liang

of fossils also indicates open, photic, and nutrient-rich waterconditions.

Mf4 (Figs. 5B and 6E–G) and Mf5 (Figs. 5C and 6H) are bothmarked by many reef-building organisms, but they differ in the typesof fabric and organisms they contain. The pebble-sized red algae andcalcite-spar in Mf4 indicate a high–wave energy environment, whilethe greater proportion of lime mud than corals in Mf5 leads to aninterpretation of a moderate– to low–wave energy environment.

The intraclasts and lime mud with moderate- to low-level fabricmaturity in Mf6 (Fig. 7A) indicate that it was deposited behind thereef and sand shoal barrier, where water circulation condition wasgood. Mf7 comprises mostly micritic matrix and peloids (Fig. 7B)that were deposited in a moderate-energy lagoonal environmenton the platform interior.

Large oncoids and fossils are the distinct features of both Mf8(Figs. 5D and 7C) and Mf9 (Figs. 5E and 7D). The emergence of onc-oids and argillaceous stripes together with the increase in echinoidand bryozoan fossils can be interpreted as products of a deep openplatform environment due to marine transgression.

The high mud-to-grain ratio and the emergence of argillaceousstripes in Mf10 (Fig. 7E) indicate that it was formed in a low-en-ergy lagoonal environment within the platform interior. The darkgray color and massive structure of Mf11 (Fig. 5F) suggest a relativedeepwater and reductive environment under a transgression

litage Formation in the Tazhong Uplift.

Fig. 5. Photographs of the cores: (A) Oolitic grainstone (Mf1), T3; (B) Red algae framestone (Mf4), T11; (C) Coral bafflestone (Mf5), T3; (D) Argillaceous bioclast intraclastpackstone (Mf8), T1; (E) Oncoid packstone (Mf9), T8; (F) Dark argillaceous mudstone (Mf11), T3; (G) Thin-bedded argillaceous-striped mudstone (Mf12), T8; (H) Mudstone-boundstone with fenestral structures (Mf13), T8.

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setting. Mf12 contains gray mudstone with argillaceous stripesthat form laminar beddings (Fig. 5G), indicating a restricted lagoonenvironment with very low wave energy and poor water circula-tion. Mf13 is a typical intertidal–supratidal deposit, given its bind-ing structure and the development of fenestral pores and laminas(Figs. 5H and 7F).

5.2. Microfacies associations characteristics

Four types of microfacies associations (MA1–MA4) were identi-fied based on microfacies stacking patterns that reflect correlativeenvironments; each association represents a distinct facies succes-sion organized within a fourth-order sequence (Fig. 8).

MA1 (Fig. 8A) includes several microfacies that were depositedin moderate- to high-energy conditions, forming the main parts ofthe platform reef and shoal complex. The gamma ray curve

associated with MA1 is shaped like a funnel with a very low valueof gamma ray logging (5–25, API units) and relatively smoothedges. The intraclastic packstones (Mf6) at the bottom of theMA1 sequence represent a moderate- to low- energy subtidal envi-ronment that developed after a minor marine transgression. Thesucceeded coral bafflestones (Mf5) and algae framestones (Mf4)were generated in a subtidal photic zone with moderate to highwave energy, while echinoid grainstones (Mf3) and ooid intraclastgrainstones (Mf1) on the top terminated the growth of reef as a re-sult of a relative drop in sea level. These microfacies are thus orga-nized in a shallowing-upward fourth-order sequence, with anincrease in energy toward the top. This association (MA1) reflectsdeposition on an open platform or a platform margin, whereby reefand carbonate sand shoal facies make up the majority of the cycle.The HST of SQ2 in wells numbered T11, T5, T6, and T3 show abun-dant MA1 (Fig. 9).

Fig. 6. Photographs of microfacies: (A) Oolitic grainstone (Mf1), note the mouldic pore, T3; (B) Intraclast grainstone (Mf2), T1; (C) Intraclast bioclast grainstone (Mf2), T11;(D) Echinoid debris grainstone (Mf3), T6; (E) Framestone (Mf4), bryozoan fossil, T3; (F) Framestone (Mf4), red algae fossil, T3; (G) Framestone (Mf4), red algae fossil, T3; (H)Bafflestone (Mf5), coral fossil, T3.

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MA2 (Fig. 8B) comprises several microfacies that were depos-ited in low- to moderate- to high-energy conditions forming car-bonate sand shoals and open platform facies. The shape ofgamma ray curve associated with MA2 is more like a funnel thanthat of MA1 because it has a wider range of gamma ray logging val-ues (5–45, API units) and relatively jagged lower edges (Figs. 8 and9). The argillaceous mudstones (Mf11) and wackestones with bio-clasts (Mf10) at the base is a result of the low-energy subtidal envi-ronment as a result of quick rise of sea level. Intraclast bioclastpackstones (Mf6) are present in the middle of the cycle, indicatinga moderate- to high-energy shallow marine environment. Theechinoid grainstones (Mf3) and bioclast intraclast grainstones(Mf2) developed on the top and are considered to be depositedin shallow and high-energy subtidal environments. The microfa-cies succession of MA2 entirely records a shallowing-upward trendafter a minor marine transgression in both the platform margin

and the platform interior, representing carbonate sand shoal andopen platform facies. MA2 frequently appears in the HST of SQ2in wells numbered T2, T7, T3, and T1 (Fig. 9).

MA3 (Fig. 8C) is featured by types of microfacies related to a rel-atively deep platform environment under extensive marine trans-gression. The shape of the gamma ray curve associated with MA3 isa funnel with the widest range of gamma ray logging values (5–60,API units), and its jagged edges are attributable to rich argillaceouscomponents (Figs. 8 and 9). The dark marly mudstone (Mf11) andwackestone (Mf10) that occur at the bottom are the record of amoderate marine transgression. The packstones with many largeoncoids and rudaceous echinoids and bryozoan fragments (Mf9)were deposited in a flat, deep, and open platform. The proportionof oncoids decreases toward the top, on which packstones withargillaceous stripes (Mf8) generated, suggesting that the deposi-tional environment followed a shallowing trend. This microfacies

Fig. 7. Photos of microfacies: (A) Intraclast bioclast packstone (Mf6), T3; (B) Peloid packstone (Mf7), T2; (C) Bioclast packstone (Mf8), bioclasts include bryozoans, ostracodes,and mollusks, T8; (D) Oncoid packstone (Mf9), note the dark and light cortex of an oncoid, T8; (E) Wackestone (Mf10) with ostracodes and other bioclasts, T1; (F) Boundstonewith fenestral structures (Mf13), T1.

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association is the predominant succession in SQ3, representing theopen platform oncoid shoal facies. The MA3 consist of a fourth-order sequence forming the entire SQ3 or the upper region ofSQ3 in wells T3, T1, T7, T8, TZ16, and T2 (Fig. 9).

MA4 includes several microfaices that are generated in a rela-tively low – to moderate – energy conditions (Fig. 8D), indicatingrestricted lagoons and tidal-flat environments in the platform inte-rior. The shape of the MA4 gamma ray curve has both gradualchanges near the top and the bottom, with an average value of20–40 (API units) (Fig. 8). The thin-bedded mudstones (Mf12)and wackestones with bioclasts (Mf10) at the bottom were depos-ited in a restricted lagoon. The following intraclastic–bioclasticpackstones (Mf6) indicate that the depositional setting turned intoa moderate-energy subtidal environment. The laminated mud-stones and blue algae boundstones with ‘‘bird’s-eye’’ structures(Mf9) make up the upper region, and they are typical of inter-tidal–supratidal conditions. The vertical stacking pattern of thesemicrofacies reflects the change in depositional setting from a sub-tidal lagoonal environment to an intertidal and to a supratidalenvironment on the platform interior. MA4 cycles are common inSQ1 and in the TST of SQ2 in wells T7, T8, and T2.

5.3. High-frequency sequences and facies correlation

Microfacies associations and high-frequency sequences in SQ3and in the HST of SQ2 are correlated with wells T1, T3, and T11

(Fig. 9). These three wells are located along the Tazhong No.1 fault(Fig. 1) where the reef-shoal complex developed, but the correla-tion shows that the vertical stacking patterns of microfacies asso-ciations are different.

The HST of SQ2 in wells T3 and T11 is primarily characterizedby successive MA1 cycles, while that of well T1 consists mainlyof MA2 cycles. These features indicate that the high-energy reefand oolitic shoal facies tend to arise in T3–T11 areas, andmoderate-energy intraclastic shoal facies predominate in the T1area.

The SQ3 in well T11 is only about 10 m thick and comprises asingle fourth-order sequence, but it is much thicker in wells T3and T1 and contains three fourth-order sequences. The upper partof SQ3 in well T3 and the entire SQ3 in well T1 are composed ofMA3 cycles. The sedimentary facies transitioned to oncoid shoalin the T1–T3 areas during the later depositional stage of the Liangl-itage Formation. There is a clear trend of increasing amounts ofargillaceous limestone toward the Lianglitage Formation’s top asa response to transgression at the end of deposition, which causedthe carbonate platform to drown.

6. Sedimentary models

Sedimentary models for SQ3 (Fig. 10B) and the HST of SQ2(Fig. 10A) are proposed for the Tazhong Uplift based on the varia-tion of microfacies and their corresponding associations.

Table 1Microfacies descriptions and interpretation.

Microfaciescode

Microfacies description Interpretation

Mf1 Oolitic grainstone. Grains are mainly ooids with minor proportions of peloids, sand-sizedintraclasts, and are well sorted and well rounded. The ratio is up to 85%. Some bioclasts areincluded, such as echinoids and algae. (Figs. 5A and 6A)

Platform margin, shoal facies, high energy

Mf2 Intraclastic grainstone. Grains are mainly sand-sized intraclasts with small amounts of bioclasts,moderately well sorted and well rounded. The bioclasts mainly include echinoids, algae, mollusks,and bryozoans. (Fig. 6B and C)

Platform margin, shoal facies, moderate to highenergy

Mf3 Bioclastic grainstone. The components are dominated by echinoid debris, and are well sorted andmoderately rounded. Syntaxial overgrowth cementation is common. Also small amounts ofbryozoans and algae are included. (Fig. 6D)

Platform margin, shoal facies, high energy, nearreef

Mf4 Red algae framestone. The red algae makes up �50% of the organisms, and they are often pebblesized. Bryozoans, mollusks, and ostracodes make up the rest of the organisms. Bioerosion on thesurface of fossils is common. Calcite-spar is frequent between particles. (Fig. 5B, 6E–G)

Platform margin, reef facies, moderate to highenergy

Mf5 Tabulate coral bafflestone. Tabulate corals make up 30% of the organisms, and they are often pebblesized and well preserved. Small amounts of bryozoans, gastropods, and ostracodes make up the restof the organisms. They are supported by lime mud. (Figs. 5C and 6H)

Platform margin, reef facies, moderate to lowenergy

Mf6 Intraclast bioclast packstone. Intraclasts are the predominant grains, ranging from silt- to sand- togravel-sized intraclasts, bioclasts and oncoids. They are usually poorly sorted and moderately/poorly rounded. Corals, algae, bryozoans, and mollusks are the main organisms. (Fig. 7A)

Platform interior, open platform facies, behind thereef-shoal facies, moderate energy

Mf7 Peloid packstone. Peloids are the most abundant components. Subordinate components includesmall ostracode fragments. The micritic matrix has a dark color. (Fig. 7B)

Platform interior, lagoon facies, moderate energy,subtidal

Mf8 Argillaceous bioclast packstone with oncoids. Echinoids and bryozoans are the main skeletoncomponents. Intraclasts and centimeter-sized oncoids occur. Argillaceous strips are present withinthe lime mud matrix. (Figs. 5D and 7C)

Platform margin, shoal facies, subtidal, highenergy

Mf9 Argillaceous oncoid packstone. This facies is dominated by large oncoids. Subordinate componentsare millimeter-sized echinoid and bryozoan debris. (Figs. 5E and 7D)

Platform margin, shoal facies, subtidal, highenergy

Mf10 Bioclast intraclast wackestone with argillaceous stripes. Most grains are silt-sized intraclasts,peloids, and bioclasts, including ostracodes, bryozoans, algae, brachiopods, and coral. (Fig. 7E)

Platform interior, lagoon facies, subtidal, lowenergy

Mf11 Mudstone, marly mudstone with a dark gray color. A few ostracodes and gastropods are present.(Fig. 5F)

Platform interior, deep open platform, below fair-weather wave base, low energy

Mf12 Thin-bedded mudstone with argillaceous stripes. (Fig. 5G) Platform interior, restricted lagoon, very lowenergy

Mf13 Blue algae boundstone, wackestone, and mudstone with a laminar and bounded structure. Fenestralpores are common. (Fig. 5H)

Platform interior, tidal facies, inter-supratidal,low energy

Fig. 8. Four microfacies associations that developed at the platform margin and in the platform interior. These microfacies associations represent different facies transitiontrends and fourth-order sequences. The value of gamma ray logging data is in API units.

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Fig. 9. Correlation of three wells along the platform margin showing the microfacies associations, depositional environments, facies distributions, and fourth-order sequencesof the Lianglitage Formation on the Tazhong Uplift.

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During the HST of SQ2 (Fig. 10A), a high-energy reef-shoal faciesbelt (wells T1, T3, T5, T6, and T11) formed a barrier at the platformmargin along the Tazhong No. 1 fault, such that open platform fa-cies (wells T2 and T7) and tidal flat and lagoon facies (T8) made upthe platform interior. The T11–T3 area is dominated by MA1,which contain coral and red algae reefs; the T1 area is dominatedby intraclastic shoals without a reef. The T2–T7 area is character-ized by intraclastic packstone that represents open platform faciesbehind the platform margin. The carbonate sand shoal facies arethe preponderant facies along the Late Ordovician platform marginfrom both the vertical and lateral aspects, with cycles that alter-nately generated �10 m of reef. Reefs are better preserved in theT3 and T11 areas toward the southeast, while the carbonate sandshoal facies prevail towards the northwest region. The T8 area,consisting of mostly MA4, suggests that tidal flat and restricted la-goon facies developed in the inner platform.

The detailed sedimentary environment of the SQ3 changed dra-matically compared with the HST of SQ2 (Fig. 10B). Many wellsshowed that the argillaceous oncoid packstones (Mf9) and argilla-ceous bioclast packstones with oncoids (Mf8) became the predom-inant microfacies types in SQ3, indicating a relative deepwaterplatform environment. As a result, the depositional facies of SQ3were uniformly distributed instead of having a significant distinc-tion from the platform margin to the platform interior during theHST of SQ2. Only the T11–T5 area is characterized by intraclasticpackstones and grainstones, which are deposited undermoderate-depth to shallow platform conditions.

7. Discussion

7.1. The significance of microfacies associations and high-frequencysequences

Previous studies on the Lianglitage Formation have confirmedthat the high-energy reef and shoal facies serve as the premiumhydrocarbon reservoir in the Tazhong Uplift (Zhao et al., 2007;Liu et al., 2004). Microfacies analysis of the formation suggests thathigh-energy microfacies (Mf1–Mf4) tend to assemble at the upperpart of the MA1 and MA2 cycles just below the fourth-order se-quence boundary, resulting from normal regression. Dissolutionof aragonitic grains and bioclasts may occur by meteoric diagenesisand produce considerable moldic porosity in reservoirs. The se-quences and facies correlation and the sedimentary models showthat high-energy microfacies successively developed in the south-eastern part of the platform margin, but they were frequentlyinterrupted by low-energy microfacies that were usually formedat the base of a high-frequency sequence in the northwestern part.These differences may be attributed to varying carbonate growthrates and differential activities of the Tazhong No.1 fault duringdeposition. The sedimentary features probably allow the south-eastern part of the platform margin to have better reservoirpotential than the northwestern part, and the favorable reservoirsin the northwestern part are more strictly confined by the high-frequency sequence.

Fig. 10. Sedimentary model of the Lianglitage Formation on the Tazhong Uplift: (A) Model for the highstand systems tract (HST) of the middle depositional sequence (SQ2);(B) Model for the upper depositional sequence (SQ3). Note the changes in main microfacies associations and the microfacies distribution between the two models.

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7.2. The evolution of the sedimentary environment and its impact onreservoir quality

The sedimentary model for the HST of SQ2 is more like the mod-els proposed by Tucker and Wright (1990) and Wilson (1975),while the model for SQ3 is dramatically different, which we believeis a result of the extensive marine transgression that occurred dur-ing the Late Ordovician. There are several reasons for this judg-ment: (1) the types and characteristics of microfacies changed

abruptly at SQ3. For example, the argillaceous content increased,large oncoids emerged, and the rock fabric maturity was lowerthan it was in SQ1 and SQ2. (2) We identified the distinctivemicrofacies association in which argillaceous oncoid packstones,wackestones, and dark marlstones were generated. (3) The in-crease in echinoderms and bryozoans indicates a much deeperwater environment (Flügel, 2004). The model for the SQ3 of theLianglitage Formation may be used as an example for studyingthe process of a drowning carbonate platform. The evolution of

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facies distribution during the marine transgression of SQ3 intensi-fied the heterogeneity of reservoirs in the Lianglitage Formation atthe Tazhong Uplift, especially in the northwestern part of the up-lift. Based on the analysis of the sedimentary facies, the reservoirsare therefore better preserved in the southeastern part of the plat-form margin.

8. Conclusions

This research involved an integrated analysis of cores, thin sec-tions, and well-logging data in order to study the microfacies anddepositional environment of the Lianglitage Formation that wascreated during the Late Ordovician in the Tazhong area of theTarim Basin in Northwest China. The conclusions are summarizedas follows:

(1) Thirteen microfacies (Mf1–Mf13) were identified within theLianglitage Formation of the Tazhong Uplift in the TarimBasin. These microfacies are distinguished by textures,structures, grains (types, sizes, and sorting), fossil types,and argillaceous content. Each type of microfacies reflectsa specific depositional environment with a certain level ofwave energy.

(2) Four microfacies associations (MA1–MA4) were used togroup specific ordered microfacies successions; thesemicrofacies associations reflect a series of correlative deposi-tional environments. Each microfacies association is orga-nized in a shallowing-upward forth-order sequencerepresenting a depositional facies, such as reef-shoal faciesat the platform margin, oncoid shoal and carbonate sandshoal facies on open platforms, and lagoon and tidal flatfacies in the platform interior.

(3) The sedimentary model for the HST of SQ2 is a regularrimmed platform system, which is characterized by reef-shoal (MA1) and carbonate sand shoal (MA2) developingalong the platform margin as a barrier, and lagoon and tidalflats (MA4) in the platform interior. However, the sedimen-tary model for SQ3 is distinct as a result of extensive trans-gression. Most of the platform became dominated by oncoidshoal facies (MA3), and the platform margin zone becameless clear than it was during the HST of SQ2. The reservoirsin the southeastern part of the platform margin are of thebest quality based on the high-frequency sequence and sed-imentary facies.

Acknowledgments

This work was supported by the National Science Foundation ofChina (Grant No. 41130422) and National Key Basic ResearchProject (No. 2011CB201100-03). We would like to thank the PetroChina Tarim Oilfield Company for data support. In addition, thecomments and suggestions by the editors and reviewers weregreatly appreciated.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jseaes.2014.01.002. These data include Google maps of the most importantareas described in this article.

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